The invention is related to a hammer drill having a coupling which is engaged for generating movement to a tool spindle in the drill when the tool is against a work surface.
A motorised hammer drill is already known from German Patent Publication DE-OS 35 06 695 (corresponding to U.S. Pat. No. 4,719,976), the hammer unit of which can be switched off by means of a coupling between a motor-driven intermediate shaft and the motion conversion gear, which produces the reciprocating piston movement. The coupling force is amplified by a lever which is loaded by the reaction forces of the hammer unit. The amplifying force applied to the coupling is absorbed by a pin in the housing and a bearing of the intermediate shaft, which is supported in a special metal bearing bridge. The effectiveness of the force amplification can be reduced by tolerances of the lever, the bearing pin and the bearing bridge. With very adverse conditions during operation, the generation of heat can cause yield of the plastic housing and thus a displacement of the pin. Moreover, any undesirable reduction of the pressing force with such lever designs can lead to a lengthening of the engagement path.